1. Field of the Invention
The present invention relates to an organic electroluminescent device and a panel for use in a plane light source and display device.
2. Description of the Related Art
Electroluminescent devices (hereinafter simply referred to as xe2x80x9cEL devicesxe2x80x9d) are favorably useful for a plane display device of self-light-emission type. Among those, unlike inorganic EL devices, organic EL devices are not required to drive with AC at a high voltage. In addition, they can be easily multi-colored due to the variety of organic compounds. Thus, their applications to a full-color display and so forth are expected and greatly studied to develop a structure with a high brightness at a low voltage.
The inorganic EL device causes light emissions of electric field exciting type. On the other hand, the organic EL device operates by injecting holes from the anode (positive electrode) and electrons from the cathode (negative electrode), and causes light emissions of carrier injection type. Positive and negative carriers injected from both electrodes respectively travel toward their opposite electrodes to generate excitons from their recombination. The light emissions of the organic EL device are ones emitted when the excitons are relieved.
A high-purity single crystalline anthracene has been used to study the organic EL device. Anthracene has low brightness and light emission efficiency values while it requires a high voltage application, and is poor in stability.
C. W. Tang et al of Eastman Kodak Co. reported in 1987 that they could achieve a stable light emission with a high brightness at a low voltage with a two-layered laminate structure of organic thin films. Their device had an organic layer, consisting of a two-layered laminate structure with a light emission layer and a hole transportation layer, which was sandwiched between a pair of electrodes and exhibited an unprecedented excellent property of 1,000 cd/cm2 at an applied voltage of 10 V (Tang et al, Appl. Phys. Lett., 51, 913 (1987)). Since then, the research and development of organic EL devices was sharply activated.
An electron transportation layer may recently be formed between the cathode and the light emission layer in addition to the light emission and hole transportation layers. A hole injection layer may also be formed between the hole transportation layer and the anode. Some light emission materials such as tris-(8-hydroxyquinolinol)aluminum (hereinafter referred to as Alq) represented by the following structural formula (1) may also serve as electron transportation materials. 
On the other hand, there are materials that serve as hole transportation and light emission materials. The light emission materials include the above-mentioned aluminum complex and distyrylarylene derivative {structural formula (2)}. The hole transportation materials include: diamine compounds such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (hereinafter referred to as TPD) {structural formula (3)}; and N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(xcex1-naphthyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine(hereinafter referred to as xcex1-NPD) {structural formula (4)}, and poly(vinylcarbazole). 
The electron transportation materials other than Alq include 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4,-triazole (hereinafter referred to as TAZ) {structural formula (5)}. 
The hole injection materials include 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine (hereinafter referred to as m-MTDATA); and phthalocyanines such as copper phthalocyanine.
Materials that form various organic layers are not limited to the above-mentioned materials, and various organic compounds have been developed for the purpose of achieving high brightness and high efficiency. After a long period of continuous driving, however, such phenomena are observed that a dark spot without a light emission ability is generated and grown even in the organic compound of high brightness and efficiency and a drive voltage must be elevated.
There are possible reasons for the above-mentioned phenomena: the organic materials are thermally degraded; and the electrode and organic layer have a low adhesive property therebetween. Materials useful for the electrode in the organic EL device include metal oxides such as ITO (indium-tin oxide), metals such as gold, silver, magnesium, lithium and aluminum, and their alloys. Other electrically conductive polymers and various semiconductor materials may also serve as the electrode in the organic EL device.
An extremely thin film typically with a thickness of 10-100 nm is used for each organic layer that composes the organic EL device. Therefore, in order to achieve a dense film structure without pinholes, such a material is preferred, which has a high glass transition point (Tg) and excellent amorphous property. A high Tg material for use in the film can increase the thermal stability of the film and improve the durability and thermal resistance of the film for a long period of driving. However, the adhesion of the film to the electrode is not directly correlated with its Tg.
The electrodes for use in the organic EL device mainly use the above-described metal oxides or metals that are typically hydrophilic. On the other hand, the organic materials that are in contact with the electrodes in order to exchange and transport carriers are hydrophobic. Therefore, the interface between the electrode/organic layer can not have a sufficient adhesion property.
As described above, the organic EL device is quite disadvantageous in that the brightness would be lowered due to the generation and growth of dark spots and the power consumption would be increased due to a drive voltage increase after continuous driving.
The present invention has been made in consideration of such disadvantages and has an object to provide an organic EL device and a panel with its improved adhesion property between an electrode and an organic layer. The device can also be prevented from decreasing its brightness due to the generation and growth of dark spots and from increasing its power consumption due to the elevation of a drive voltage. The present invention also provides a process of manufacturing the device and panel.
The inventors experimented and studied a lot to find out the structure of an organic EL device that can suppress the generation and growth of dark spots and simultaneously have a less amount of elevation of the drive voltage. As a result, the inventors found an approach to eliminate the above-mentioned disadvantages and finally reached to the present invention. The approach is to introduce an interaction caused between an organic compound having a sulfur-atom-containing substituent in a specific structure and the surface of a specific material into an interface between electrode/organic layers in an organic EL device.
The specific material includes metal oxides such as ITO (tin-doped indium oxide), ATO (antimony-doped tin oxide), FTO (fluorine-doped tin oxide), AZO (aluminum-doped zinc oxide), and metal oxides such as tin dioxide and zinc oxide. It also includes metals selected from the group consisting of copper and noble metals (gold, silver, platinum group (platinum, ruthenium, rhodium, palladium, osmium and iridium)), their alloys, and semiconductor compounds.
The semiconductor compounds include a p-type semiconductor compound for use in a hole injection (transportation) electrode and an n-type semiconductor compound for use in an electron injection (transportation) electrode. The p-type semiconductor compound includes a material mainly consisting of either compound of metal chalcogenide, metal halide and metal carbide. In further detail, the p-type semiconductor compound includes a material mainly consisting of either compound of nickel oxide, copper oxide, lead oxide, rare earth oxide, copper iodide and lead sulfide.
The compound having a sulfur-atom-containing substituent in a specific structure includes a substance that contains a sulfur-containing group such as a mercapto group (xe2x80x94SH), a sulfide group (xe2x80x94Sxe2x80x94) or a disulfide group (xe2x80x94Sxe2x80x94Sxe2x80x94) in its molecular structure. The mercapto group (xe2x80x94SH) is, however, more preferred. Among them, an alkyl mercapto group (xe2x80x94(CH2)nSH) (n is an integer of 1-10) is most preferred. The organic compound that includes such structures partially in its molecular structure can bond at its sulfur atom position or positions to the surfaces of such materials with strong interaction as metal oxides including ITO, metals selected from the group consisting of copper and noble metals (gold, silver, and platinum group), their alloys, and semiconductor compounds. This is schematically shown in FIG. 1.
This interaction is much stronger than that of an LB film (Langmuir-Blodgett film) formed on the surface of a substrate by the so-called physisorption. (See, for example, Kondo et al, xe2x80x9cBunsekixe2x80x9d, p457 (1997); and K. Kajikawa et al, xe2x80x9cMolecular Electronics and Bioelectronicsxe2x80x9d, 7(1), 2 (1996)).
The molecular structure that contains a sulfur atom is not particularly limited to the above exemplified three sustituents but can be extended to other structures that would act in the same way as the molecular structure.
The organic EL device of the present invention comprises at least one organic compound thin film including a light emission layer between two electrodes. In its aspect, a super thin film composed of an organic compound having a sulfur-atom-containing substituent in a specific structure is introduced onto the light-emission-layer side surface of at least one electrode. That is, such a super thin film may be introduced into the interface between the anode and the hole transportation layer, the interface between the cathode and the electron transportation layer, or both of the above-mentioned interfaces.
Processing the surface of the electrode forms the super thin film. The electrode whose surface is reformed by introducing the super thin film can be selected among metal oxides such as ITO (Indium Tin Oxide), metals selected from the group consisting of copper and noble metals (gold, silver, and platinum group), their alloys, and semiconductor compounds, as described above. In particular, ITO, gold, silver and so forth have often been used as electrodes in organic EL devices. Therefore, the procedure of the present invention can be widely applied to the results obtained through the previous research and development.
The organic EL device typically uses a material with a high work function for the anode and a material with a low work function for the cathode. The materials for use in the anode may include such metal oxides as ITO, ATO, FTO, AZO, tin dioxide and zinc oxide. The materials for use in the cathode may include metals such as sodium, magnesium and indium, and alloys such as a magnesium-silver alloy and a lithium-aluminum alloy. Materials with relatively lower work functions are preferred for use in the cathode. ITO may be used in the cathode and gold, in the anode, for example.
The organic EL device comprises at least one transparent or translucent electrode to take light out of it. In application to typical displays, it is required to use a transparent or translucent electrode only at one surface side. Therefore, it is also required to take light out of the organic EL device of the present invention without a large loss using at least one transparent or translucent electrode. New sentences are inserted therein.
The structure of a compound having a sulfur-atom-containing substituent for use in the present invention can be directed to a preferred structure according to the material of a carrier transportation layer formed on the electrode. The conception will be explained in more detail. On one hand a compound which has a similar structure to that of a material for use in a hole transportation layer and which has a sulfur-atom-containing substituent is used between the hole transportation layer and the anode. This causes a high affinity between the hole transportation layer and the anode. On the other hand a compound which has a similar structure to that of a material for use in an electron transportation layer and which has a sulfur-atom-containing substituent is used between the electron transportation layer and the cathode. This causes a high affinity between the electron transportation layer and the cathode. These phenomena result from two facts; the interaction between the above-mentioned sulfur atom and the electrode composed of a specific material, and a higher affinity between molecules having similar structures. It is, however, required to design the sulfur-atom-containing substituent so that the substituent may not hinder the above-mentioned interaction due to its configuration.
The sulfur-atom-containing substituent with a configuration that causes a strong interaction with the electrode may include the previously described mercapto, sulfide and disulfide groups. These substituents are expected to achieve a higher adhesion property as the number of those groups increases, but can not be introduced too much to synthesize.
According to the present invention, an organic material includes a molecular skeleton that contains a sulfur atom capable of strongly bonding with the electrode surface and also a skeleton that has a high affinity for the organic layer. The material is interposed as a super thin film into an interface between the electrode and the organic layer. Thus, the adhesion property between the electrode and the organic layer can be improved. Therefore, an organic EL device and organic EL panel with a high durability and less increase in power consumption as well as a process of manufacturing the device and the panel can be provided.